Further research and developments of laser devices directed to achieve a higher power, a shorter pulse, and a shorter wavelength of output light have been made. Among the aforementioned devices, a Q-switch laser device using a Q-switching technology has received a lot of attention from where output pulse laser light with a high power and a short pulse could be obtained. In the Q-switching technology, with a configuration including not only a gain medium but also a Q-switch element in a resonator, the laser oscillation is controlled by a varied Q-value of the resonator determined by the Q-switch element laser oscillation, thereby the output laser light with a short pulse and a high power could be provided.
Various kinds of Q-switching technologies have been known. Among the aforementioned technologies, a passive Q-switching laser technology using a saturable absorber as a Q-switch element is more preferable in terms of a simple and compact configuration of a laser device than the other Q-switching technologies. The saturable absorber absorbs less incident light power when the incident light power gets increased. The saturable absorber absorbs the incident light when the power of the incident light is equal to or less than an absorption saturation threshold value. When the power of incident light reaches or is higher than the saturation absorption threshold value of the saturable absorber, the saturable absorber turns in to a transparent one. Using such features, the saturable absorber is used as the Q-switch element.
Specifically, the passive Q-switch laser device using the saturable absorber as the Q-switch element operates in a manner described below. Upon excitation of the gain medium, atoms are excited to a laser upper level of the gain medium. The excited atoms remain at the laser upper energy level during a laser upper energy level life time period τ and then transit to a laser lower energy level after the life time τ followed by fluorescence emission. Population inversion of the gain medium is initially small at the start of the excitation of the saturable absorber, thus the fluorescence intensity emitted from the gain medium and incident power on the saturable absorber is small. Therefore, the saturable absorber is not transparent initially at the start stage of the excitation of the gain medium, and the Q-value of the resonator is small, resulting in no laser oscillation. Since the gain medium is continuously excited even during a period in which the Q-value of the resonator is small and the laser oscillation does not occur, the population inversion of the gain medium gradually increases leading to gradually increased the power of the fluorescence emitted from the gain medium and incident on the saturable absorber. Finally upon excess of the power of the fluorescence emitted from the gain medium and incident on the saturable absorber over the absorption saturation threshold value of the saturable absorber, the absorption by the saturable absorber abruptly decreases and the saturable absorber becomes transparent. Meanwhile, the Q-value of the resonator is increased, and the induced emission is abruptly promoted in the gain medium. As a result of aforementioned process, laser oscillation occurs. However, the induced emission is promoted at once due to the excessively high gain. The accumulated population inversion is consumed in short time leading to automatically end of the oscillation in short time. Thus, the pulse laser light possessing a high power and a short pulse is generated from the resonator.
Various kinds of gain media as well as various kinds of excitation units that excite the gain medium, are known. For example, an Nd:YAG crystal is used as the gain medium, and a semiconductor laser light source is used as the excitation unit for exciting the gain medium through excitation light irradiation. In this case, the excitation light from the semiconductor laser light source is applied to the Nd:YAG crystal which is served as the gain medium. Nd ions contained in the gain medium are excited to an upper level, thereby causing population inversion (for example, see Japanese Laid-Open Patent Publications Nos. 2003-86873 and 2003-198019).